This invention relates to an index type color picture tube, and more particularly a composition of phosphors constituting its fluorescent screen.
FIG. 1 shows a structure of the screen of a one-electron-beam, index type color picture tube. On the rear surface of a front glass 10 are coated red, blue and green, primary tricolor emitting phosphors 11a, 11b and 11c (hereinunder called color phosphors) in vertical stripes with carbon black coated also vertically therebetween as guard bands 12. Over the color phosphors and guard bands is vapor-deposited aluminum to constitute a metal back 13, upon which are further coated postion detecting phosphors 14 (index phosphors) in vertical stripes.
The guard bands 12 are positioned alternately with the stripes at the respective color phosphors so as to be separation bands for preventing color contamination caused by an electron beam exciting two color phosphors simultaneously. The reflection of external light impinging on the front surface of the screen can be reduced as well and there is additionally an effect of improving contrast.
The position detecting phosphors 14 are formed on the metal back corresponding to the guard bands so that an electron beam is not hindered from reaching the respective color phosphors.
As an electron beam from a single electron gun 15 is scanned on the screen, the index phosphors are excited together with the color phosphors and emit light. The light emanated is led out through an index detecting window 16 in the rear funnel of the picture tube and converted into electric signals by an index light detector.
The electric signals provide information pertaining to the relationship between the scanning positions of the electron beam and the positions of the stripes of the color phosphors. The electric signals are pulse signals, too, having a period determined by the pitch of the stripes of the position detecting phosphors, and chrominance signals for red, blue and green are switched by position signals to be supplied to the electron gun in order to excite the stripes of the respective color phosphors and to reproduce a color image.
As described above, the index type color picture tube as shown in FIG. 1 comprises tricolor stripe shaped phosphor layers and a stripe shaped index phosphor layer adapted to show the position of a scanning beam. Used as the tricolor phosphors are a Y.sub.2 O.sub.3 :Eu phosphor for red color, a ZnS:Cu, Al or ZnS:Cu, Au, Al phosphor for green color, and a ZnS:Ag phosphor for blue color. Used as the index phosphor are a YAlO.sub.3 : Ce phosphor (for emitting ultraviolet rays) having a short after-glow time, a Y.sub.3 Al.sub.5 O.sub.12 :Ce phosphor (emitting green color) or a Y.sub.2 SiO.sub.5 :Cu phosphor (emitting ultraviolet rays), etc. However, with zinc sulfide type phosphors emitting green and blue colors, their brightness will generally increase substantially linearly with increase in the electron beam current within its low value range, but within the high value range, their brightness will saturate without exhibiting linear increase. Only with the conventional red color phosphor, even in the high current range, there occurs no appreciable saturation phenomenon of the brightness, and the brightness increases linearly as the electron beam current increases.
For this reason, in a color picture tube utilizing such tricolor phosphors, a white color picture becomes more reddish in the high current range than in the low current range, thus degrading uniformity of whiteness. With a shadow mask type color picture tube having three electron guns corresponding to the tricolor phosphors, this problem can be solved relatively simply by correcting characteristics of the associated electric circuits, but in the index type color picture tube without a shadow mask, having a single electron gun, it has been extremely difficult to take advantage of such a countermeasure.